The dehydration of whole fruit or fruit pieces is a well known method for preserving the fruit for ultimate manufacturing, processing or consumption. At the dehydrated moisture levels, enzymatic and chemical reactions are minimal. Therefore, dehydrated fruit has marked quality stability. For ultimate manufacturing or further processing, the increased bulk density additionally reduces shipping, handling and packaging costs. The dehydrated fruit can be consumed directly or combined with other consumable ingredients such as cereals, fruit mixtures, food mixes or the like. The dehydrated fruit can be stored at room temperature for extended periods. When properly processed, the dehydrated fruits should retain quality attributes, i.e. color, flavor and texture. When they are reconstituted, the fruit should be of a quality approaching that of fresh fruits of the same types.
Dehydrated fruit is contrasted from dried fruits which normally have a moisture content of between 10-20%. As used herein, dehydrated fruits are those which are dried to a moisture content of 5% of lower. The moisture content of fruits and vegetables determine, to a large extent, their physical qualities of crispness and/or flexibility.
Dried or concentrated fruits of intermediate moisture content, such as raisins and apricots, are produced in a number of ways. Generally, the drying is accomplished at atmospheric pressure using a combination of convection and conduction heating. To achieve the requisite drying, high fruit temperatures are required to remove the fruit moisture. These elevate temperatures can cause irreversible changes in the fruit color and flavor, oftentimes producing characteristics commonly called "dried fruit taste." Such drying may require additions of sulfur dioxide and other preservatives to prevent excessive color changes and/or deterioration during drying and storage.
Even more difficult is the complete dehydration of fruit. Such dehydration has conventionally been approached on the basis of using even higher temperatures or heating under vacuum to remove the remaining 15-20% of the moisture from the dried fruit. Most commonly, the fruit is heated by conduction in direct contact with the heated surface, such as a drum. This requires extremely long heating cycles ranging between 6-24 hours to effect the necessary moisture removal. This process removes the characteristic taste of the fruit by driving the organic fragrance compounds from the fruit with the water vapor. Such a process is not desired and is not particularly amenable to continuous operations. Furthermore, the product may be changed structurally, being shrunken in appearance, hard in texture and lack fresh fruit flavor. Freeze drying under vacuum has been used to provide the drying with improved properties. However, this requires high vacuum with low evaporative rates resulting in a slow process with prohibitive capital and operating costs for most foodstuffs. This slow processing reduces the taste of the fruit.
The limitations of surface heating and evaporation may be explained by understanding the accepted model for such a process. For given processing conditions, there is an initial constant rate period wherein evaporation proceeds at a constant rate until the fruit surface becomes unsaturated. This moisture removal is by capillary action or diffusion and is primarily effected through the heating of the external surface of the fruit., For further moisture removal, the surface heat energy must be conducted inwardly to the cool interior of the fruit core to cause migration of the moisture across a negative moisture gradient from high moisture density internal areas to the partially dehydrated surface. This results in a falling migration rate stage where the evaporative rate decreases steadily. For thorough dehydration, a third rate period or stage is experienced wherein moisture has to be evaporated from the fruit. This rate is even slower and requires a product temperature well above those which would be expected for evaporation at the prevailing drying vessel pressure.
It has been proposed to use microwave or radiation under vacuum conditions for the processing of various foodstuffs and products. As disclosed in U.S. Pat. Nos. 4,045,639, 4,015,341 and 4,229,886, microwave radiation may be used for dehydrating various moisture containing materials in a evacuated chamber. U.S. Pat. Nos. 4,096,283 and 4,341,803 disclose that a partially dried food product can be batch dehydrated at an intermediate processing using microwave energy. Use of microwave in zones for fruit drying is disclosed in U.S. Pat. No. 4,640,020 and is a relevant showing of microwave as it was developed during the project resulting in the present invention.
However, microwave energy alone does not provide a totally effective and acceptable method for dehydrating food, particularly fruit and fruit pieces, as disclosed in the introductory portion of U.S. Pat. No. 4,746,968. While microwave radiation, as shown in U.S. Pat. No. 4,640,020, is effective to internally raise the temperature of the fruit piece or particle and promote outward migration of the moisture, the outer surface, which may be skin in a whole fruit such as a grape, is not effectively heated. The evaporation rate is effectively limited by the ambient temperature and vacuum conditions; therefore, the rate of microwave energy which can be effectively employed is limited. Unless the migration rate effected by the microwave heating of the interior of the fruit piece is below the evaporative rate determined by the ambient temperature of the vacuum chamber, the moisture can recondense on and be absorbed back into the fruit piece. This can cause the subcooling of the surface further reducing the effectiveness of the microwave heating of the interior of the fruit.